This invention relates generally to spin valves. More particularly, it relates to the coupling field of spin valves.
A spin valve or a magnetoresistive (MR) sensor detects magnetic field signals through the resistance changes of a read element, fabricated of a magnetic material, as a function of the strength and direction of magnetic flux being sensed by the read element. The conventional MR sensor operates on the basis of the anisotropic magnetoresistive (AMR) effect in which a component of the read element resistance varies as the square of the cosine of the angle between the magnetization in the read element and the direction of the flow of sense current through the read element. Such a MR sensor can be used to read data from a magnetic medium. An external magnetic field from the magnetic medium (the signal field) causes a change in the direction of magnetization in the read element, which in turn causes a change in resistance (xcex94R/R) in the read element and a corresponding change in the sensed current or voltage.
A spin valve has been identified in which the resistance between two uncoupled ferromagnetic layers varies as the cosine of the angle between the magnetizations of the two layers and is independent of the direction of current flow.
An external magnetic field causes a variation in the relative orientation of the magnetization of neighboring ferromagnetic layers in a spin valve. This in turn causes a change in the spin-dependent scattering of conduction electrons and thus the electrical resistance of the spin valve. The resistance of the spin valve thus changes as the relative alignment of the magnetizations of the ferromagnetic layers changes.
Typically, a conventional simple spin valve comprises a ferromagnetic free layer, a spacer layer, and a single-layer pinned ferromagnetic layer, which is exchange-coupled with an anti-ferromagnetic (AF) layer. In an anti-parallel (AP) pinned spin valve, the single-layer pinned ferromagnetic layer is replaced by a laminated structure comprising at least two ferromagnetic pinned sublayers separated by one or more thin non-ferromagnetic anti-coupling sublayers.
In general, the larger the value of xcex94R/R and the smaller coupling the field Hf, the better the performance of the spin valve. The xcex94R/R value of a spin valve conventionally increases as the thickness of the spacer layer decreases due to reduced shunting of the sense current in the spacer layer of the spin valve. For example, a spin valve with a copper spacer layer having a thickness of 28 xc3x85 will achieve a xcex94R/R of about 5%. If the thickness of copper spacer layer is reduced to 20 xc3x85, a xcex94R/R of 8% will be obtained. However, the ferromagnetic coupling field Hf also increases as the thickness of the spacer layer decreases. In addition, the ferromagnetic coupling field of conventional spin valves is unstable upon annealing cycles. For example, the ferromagnetic coupling field of spin valves changes from about +5 Oe at the beginning of the annealing process to +20 Oe after annealing cycles.
An article entitled xe2x80x9cOxygen as a Surfactant in the Growth of Giant Magnetoresistance Spin Valvexe2x80x9d published Dec. 15, 1997 by Journal of Applied Physic to Egelhoff et al. discloses a method for increasing the giant magnetoresistance xcex94R/R of Co/Cu spin valves with use of oxygen. In this method, oxygen is introduced in an ultrahigh vacuum deposition chamber with an oxygen partial pressure of 5xc3x9710xe2x88x929 Torr during deposition of the spin valve layers, or the top copper surface is exposed to the oxygen to achieve an oxygen coverage, after which growth of the sample is completed. The oxygen acts as a surfactant during film growth to suppress defects and to create a surface that scatters electrons more specularly. Oxygen coverage decreases the ferromagnetic coupling between magnetic layers, and decreases the sheet resistance of spin valves.
Unfortunately, this technique requires a very small oxygen partial pressure window around 5xc3x9710xe2x88x929 Torr, since when the oxygen partial pressure is increased to only 10xe2x88x928 Torr, all GMR (xcex94R/R) gain due to oxygen is lost, and at oxygen pressures higher than this, the fall-off in GMR is rapid. This very small oxygen partial pressure is very difficult to achieve or to maintain in a large manufacturing type system. Also, oxygen exposure of only one surface of the copper spacer layer does not optimize the ferromagnetic coupling field. Furthermore, the use of oxygen for all spin valve layer depositions may result in oxidation of Mn in anti-ferromagnetic materials, such as FeMn, PtMn, IrMn, PdPtMn and NiMn, and thus kills the spin valve effect. Therefore this technique can not be applied for spin valve deposition.
In addition, adsorbing oxygen only on the copper surface does not improve the GMR, and produces only a positive coupling field. Furthermore, this technique results in decrease in sheet resistance, which reduces the overall signal. Finally, the prior art oxygen treatment does not show stabilization of the ferromagnetic coupling field upon hard bake annealing cycles.
There is a need, therefore, for an improved method of making spin valves that overcomes the above difficulties.
Accordingly, it is a primary object of the present invention to provide spin valves with low and stable coupling field Hf.
It is a further object of the invention to provide spin valves with high magnetoresistive ratio xcex94R/R.
It is another object of the invention to develop a process of making spin valves with oxygen partial pressure levels that can be used in manufacturing systems.
It is another object of the invention to develop a process of making spin valves achieving negative coupling fields in production processes.
It is a further object of the invention to develop a process of making spin valves, which does not result in reduction in sheet resistance.
It is another object of the invention to develop a process of making spin valves, which can be used with metallic anti-ferromagnetic materials or in addition to oxide anti-ferromagnetic materials.
It is an additional object of the invention to provide a method of making spin valves having above properties, which can be applied for bottom and top spin valves.
These objects and advantages are attained by spin valves having a first surface of one ferromagnetic layer and a second surface of a spacer layer, treated with oxygen.
According to a first embodiment of the present invention, a simple spin valve includes a ferromagnetic layer having a first surface, such as a ferromagnetic free layer, and a spacer layer having a second surface. One or more of the first and second surfaces has been treated with oxygen after deposition of the corresponding layer and oxygen treatment has been shut off before depositing a subsequent layer. Treatment with oxygen herein refers to exposing a surface of a layer of material to oxygen after the layer has been deposited. Physisorbed oxygen on these surfaces limits the intermixing between the layers and reduces the surface roughness of these surfaces. As a result, the coupling field is reduced. The obtained coupling field is around xe2x88x9210 Oe for about 20 xc3x85 copper, and the coupling field is stable upon hard bake annealing cycles at 232xc2x0 C. for 11 hours or at 270xc2x0 C. for 6 hours. Furthermore, the magnetoresistive ratio xcex94R/R is enhanced from about 6% to about 9%.
According to a second embodiment of the present invention, a bottom AP-pinned spin valve includes a first surface of a ferromagnetic layer, which is an AP-pinned sublayer, and a second surface of a spacer layer, treated with oxygen. The effect of oxygen surface treatment in AP-pinned spin valves is similar to the effect of oxygen surface treatment in simple spin valve as described in the first embodiment.
A method of making spin valves having surfaces treated with oxygen is described in a third embodiment of the present invention. An ion beam sputtering technique may be used to make the spin valves. A substrate is provided in a vacuum chamber. A first ferromagnetic layer, which may be a free layer for a top spin valve or a pinned layer for a bottom spin valve, is deposited onto the substrate. A first surface of the first ferromagnetic layer is exposed to an oxygen-rich atmosphere with an oxygen partial pressure of between about 1xc3x9710xe2x88x927 and about 5xc3x9710xe2x88x925, by introducing an oxygen burst into the vacuum chamber for about 30 seconds. The oxygen molecules are directed toward the substrate, and a substrate shutter is fully open to directly expose the substrate to the oxygen beam. Oxygen is physisorbed on the first surface. After about 30 seconds, the oxygen is shut off, and the normal process of fabrication of the spin valve is resumed. A spacer layer of about 20 xc3x85 thick is deposited on the oxygen treated surface. A second oxygen burst is introduced into the vacuum chamber with an oxygen partial pressure of about 5xc3x9710xe2x88x926 Torr for treating a second surface of the spacer layer. The process of treating this second surface is similar to the process of treating the first surface as described above. The oxygen is again shut off before a second ferromagnetic layer, which may be a pinned layer for a top spin valve or a free layer for a bottom spin valve, is subsequently deposited.
The method described in the third embodiment may be used for top and bottom simple spin valves, top and bottom AP-pinned spin valves, and dual spin valves.
According to a third embodiment of the present invention, spin valves of the types depicted in the first and second embodiments, which are made by the method described in the third embodiment, is incorporated in a GMR read/write head. The GMR read/write head includes a lower shield layer and an upper shield layer, which sandwich a spin valve, a lower gap disposed between the lower shield and the spin valve, and an upper gap disposed between the upper shield and the spin valve. The spin valve converts a magnetic signal to an electrical signal using the magnetoresistive effect generated by a relative angle between magnetizing directions of a ferromagnetic free layer and a ferromagnetic pinned layer.
GMR read/write heads of the type depicted in the fourth embodiment is incorporated in a disk drive system including a magnetic recording disk, a motor for spinning the magnetic recording disk, the read/write head and an actuator for moving the read/write head across the magnetic recording disk, according to a fifth embodiment of the present invention.